In a conventional edgeless speaker, a diaphragm having a flat sound radiating surface is supported by means of a damper which is fixedly connected at its other end to the frame or housing of the speaker. Since the diaphragm is supported in the direction of its essentially reciprocal movement at one location, it is apt to perform a pendulum movement centering around the supporting portion when the voice coil is energized. As the diaphragm performs such a pendulum movement, a voice coil bobbin fixedly connected to the rear surface of the diaphragm undesirably comes into contact with the magnetic circuit, resulting in the occurrence of abnormal sounds. In other words, because of the pendulum movement the direction of the movement of the diaphragm and the voice coil bobbin is not always parallel to the walls of the magnetic gap in which the voice coil bobbin is supposed to move reciprocally. In order to improve the sound quality the diaphragm is required to move only in one direction, parallel to the axis of the voice coil bobbin.
In addition, it is necessary to prevent the air behind the diaphragm from leaking outwardly toward the front side thereof to improve the sound pressure level to frequency characteristic. With the above-mentioned conventional edgeless speaker, however, the air behind the diaphragm is easily radiated toward the front side through an annular gap defined between the periphery of the diaphragm and the inner surface of the frame or housing. As a result, a dip occurs in the sound pressure level to frequency characteristic curve in the low frequency range.
Generally speaking, such a speaker diaphragm having a flat sound radiating surface is required to be light in weight, and not to change with time. In addition, it is desirable that the sound radiating surface portion of the diaphragm have a given stiffness for improvement in the sound quality. Therefore, in order to reduce weight, some conventional speakers have diaphragms made of a foam resin, such as a foam polyurethane resin, a foam acrylic resin, a form styrene resin, or the like. In order to manufacture such a diaphragm with the above-mentioned resin, a molding technique, is typically used in which a raw material of a foam resin, which is in the form of beads, is put in a mold, and the mold is heated up to a given temperature to effect foaming. However, when a diaphragm is simply made by means of a mold, such a diaphragm has high-density layers close to the surfaces thereof. Because of the high-density surface layers the total weight of the diaphragm is heavier than a diaphragm having a small and constant density throughout the entire portions thereof. Such a diaphragm having a constant density of a foam resin throughout the entire portions thereof can be manufactured by shaving the surfaces of the diaphragm made by means of a mold to remove the high-density surface layers. Of course it is also possible to form such a diaphragm by cutting a foam resin block or plate to the shape of a desired diaphragm.
However, the diaphragm made by one of the above-mentioned techniques has the following disadvantages: Namely, there are blowholes made on foaming at the exposed surfaces. Therefore, the sound radiating surface, which will be exposed to the outside of a speaker enclosure, is easily damaged. Furthermore, because of the presence of the blowholes the sound radiating surface is not smooth, resulting in poor sound reproducing characteristics. Such undulatory surfaces of the diaphragm have a poor lightproof characteristic and a poor moistureproof characteristic so that the color of the sound radiating surface changes within one or two years, and the elasticity at the same surface reduces as time goes because of air-slaking. Once the sound radiating surface of the diaphragm has deteriorated in this manner, rags come off from the surface to scatter due to vibrations of the diaphragm. Therefore, it is necessary to protect the sound radiating or front surface by attaching a thin sheet or film to the front surface which is exposed to the front portion of the speaker enclosure.
Since the stiffness at the surfaces having blowholes is relatively small, if the sound radiating surface is simply constructed of such a surface, the sound pressure level at a given frequency is abnormally low resulting in a poor frequency characteristic of a speaker.
Although it is known that a sheet of paper is adhered to the front portion of a diaphragm to protect the front surface of the diaphragm made of a foam resin and to increase the stiffness at the sound radiating surface portion, such a sheet of paper is easily detached due to vibrations and is further prone to deterioration with the passage of time.
Therefore, it is desirable to use a sheet of film made of a resin. There are two possible structures of a diaphragm having such a film of resin adhered to the flat front surface thereof, as follows: The first structure is one wherein the film is adhered to the flat front surface after directly spreading an adhesive agent on the front surface of the diaphragm. With this structure, however, the adhesive agent is unnecessarily absorbed by recesses or blowholes at the front surface of the diaphragm so that the diaphragm becomes too heavy. The other structure is one wherein the adhesive agent is spread on a surface of the film to be attached to the front surface of the diaphragm before the film is adhered to the front surface. With this structure, however, if the amount of the adhesive agent is too small, the film would be adhered to only the projecting portions in the flat front surface of the diaphragm resulting in a weak adhesive force therebetween. As a result, the film easily comes off due to vibrations. On the contrary, if the amount of the adhesive agent is too large, although the adhesive force is strong, the weight of the diaphragm would increase as much as the increase in the amount of the adhesive agent.
Apart from the above-mentioned various disadvantages, up to this time, for instance, in a cone type speaker, undulations occur in the sound pressure level to frequency characteristic curve due to the diaphragm shape, although the diaphragm vibrates reciprocally. For instance, in a speaker having vertex angle of 120 degrees and diameter of 8 inches, there is a peak of approximately 5 dB around 1 KHz, and a dip of approximately 10 dB around 6 KHz. The degrees of the peak and dip reduces as the vertex angle becomes large, and therefore, it is necessary to make the sound radiating surface flat in order to remove the influence of the shape. However, the resonance frequency of a speaker diaphragm which is flat and has a constant thickness is low compared to a cone type diaphragm. Therefore, when a flat speaker diaphragm is used, it is required to make the thickness great and also make the flexural rigidity great. In addition, it is desirable to manufacture a speaker diaphragm of a low density material because the weight of the speaker diaphragm tends to increase as the thickness increases. As such a material, a foam resin, such as a foam acrylic resin, Reticulated Vitreous Carbon (trademark of Fluoro Carbon Co.) or the like may be used. Also such a light diaphragm may be manufactured by sandwiching a disk having a honeycomb mesh structure by a pair of thin sheets of a light metal, such as aluminum. Up to this time, in order to connect the voice coil bobbin to the speaker diaphragm, an annular recess is made in the rear surface portion of the diaphragm made of a foam resin for receiving one end of the voice coil bobbin. In other words, the edge portion of the voice coil bobbin is inserted in the annular recess made in the back of the diaphragm. However, it is very difficult to make such a recess in such a manner that the center axis of the diaphragm agrees with the center axis of the voice coil bobbin. Furthermore, in case of a diaphragm having the above-mentioned honeycomb mesh structure, it is impossible to make such a recess. Therefore, in a conventional speaker having such diaphragm of honeycomb mesh structure, the voice coil bobbin is simply adhered at its one end to the rear side of the diaphragm. In such a speaker, the efficiency in vibration transmission from the voice coil bobbin to the diaphragm is poor.